Wood is finished to protect it from physical or chemical damage and natural degradation, and to enhance its natural beauty, increasing its aesthetic value. To avoid possible damage to finish coatings during assembly, wood furniture often is partially or fully assembled prior to finishing.
Coating Application Processes
Coating may be applied using either a short- or long-sequence process as described below.
Medium-and low-end manufacturers often use a short sequence consisting of three coating application steps: stain, sealer, and topcoat.
High-end manufacturers use a long sequence with at least seven application steps. Typical coatings used in the long sequences are: stain (two applications), washcoat, filler, sealer and topcoat (two applications). Overall, emissions from long sequences are significantly higher than those from short sequences, because the total number of finishing steps (i.e., the amount of coating used to finish a single item) is greater. A typical long sequence can take approximately 400 minutes to complete, of which a total of 340 minutes is used for drying the coating between applications.
Traditionally, wood finishing materials are solvent-based. Solids in the coatings are resins and pigments. These are the components that remain on the wood once the coating is dry. Generally, two types of sealer and topcoat materials are used in wood finishing: nitrocellulose and catalyzed.
Nitrocellulose Sealers and Topcoats
Nitrocellulose sealers and topcoats are used widely in wood finishing because they are easy to apply and repair, dry quickly, are familiar to the industry, and provide the final product appearance to which consumers are accustomed. Clear nitrocellulose topcoats are commonly known as lacquers. Traditional nitrocellulose coatings contain a low percentage of solids, ranging from 10 to 30 percent for sealers and lacquers.
However, three main drawbacks offset the benefits of nitrocellulose coatings: nitrocellulose is highly flammable; the coating requires use of solvents that are toxic and volatile, creating large quantities of potentially harmful emissions; and the dried finish is not highly durable--easy to damage, is ruined by water, and turns yellow when exposed to sunlight.
Catalyzed coatings are generally limited to short-sequence finishing. They are used for sealer and topcoat applications in the manufacture of kitchen cabinets and high-use furniture such as office desks and casual dining sets. Catalyzed coatings are more durable than traditional nitrocellulose coatings, exhibiting greater resistance to moisture, and physical and chemical damage.
Acid-catalyzed topcoats are commonly known as conversion varnishes. These have a lower volatile organic compound (VOC) content and a higher solids content than nitrocellulose coatings, although air emissions are still significant. Acid-catalyzed conversion coatings emit some volatile hazardous air pollutants (VHAPs),* such as formaldehyde (which is also a VOC), that are different than those emitted by nitrocellulose coatings. Inhalation exposure to formaldehyde can result in eye, nose, and throat irritation, and respiratory problems; studies have linked formaldehyde with increased cancer risk.
Stains are transparent color coats that contain some pigment to impart color and enhance the natural wood grain. They are generally methanol-based and have a very low solids content, eight percent or less.
An alternative to staining, paint contains more pigment and essentially covers the natural wood grain. A primer coat is used under a paint coat. Paints and primers are traditionally solvent-based.
Seven general categories of contact adhesives used by the wood furniture industry are:
- urea-formaldehyde resin, epoxy resin
- hot melt
- heat seal
- polyvinyl acetates (PVA)
Solvent-based adhesives are used for a wide range of laminate applications. They contain polymeric resins dissolved in a VOC solvent and are 15 to 20 percent solids and 80 to 85 percent solvents. Contact adhesives can be applied with manual spray guns or brushes, or an automated system, particularly at large facilities.
Urea-formaldehyde resin adhesives are used widely in the manufacture of particleboard and medium-density fiberboard (MDF), but they also are used to apply wood veneers and in upholstery operations. Not all of the formaldehyde in the adhesive volatilizes; however, emissions from the free formaldehyde, which does not bind into the adhesive during curing, can be significant. Formaldehyde is a VHAP and can have serious health consequences.
Epoxy resins are often used to laminate vinyl to MDF. Many epoxy resin adhesives are solvent-based; however, some are 100 percent solids. Epoxy resin adhesives are applied by manual or automated roll-coating. Solvent-based epoxies are used because they tend to have a higher initial bond strength than 100 percent solids epoxies. Epoxy resin adhesive lamination requires cure temperatures of more than 50 degrees Fahrenheit for a period of one to three days.
Solvent-based, urea-formaldehyde resin and some epoxy resin adhesives can generate significant air emissions. Hot melt, heat seal, aqueous-based, and PVA adhesives do not generate significant air emissions at wood furniture manufacturers and are considered alternatives to solvent-based adhesives. In the manufacture of partitions and fixtures and other types of wood furniture products that involve covering a composite material core with a laminate, the adhesives used can generate significant air emissions. The laminate is commonly a form of vinyl, polyvinyl chloride (PVC), coated paper, melamine, or wood veneer. Substrates are particleboard or MDF.
Quantity of coatings or adhesives used, and therefore emissions generated, correlates with the application equipment used. Generally, there are two broad types of application technologies: manual spray guns and automated finishing systems. Transfer efficiency (TE) of an application technology is the percentage of material that exits the application equipment and is deposited on the work item. The lower the TE of the application equipment, the greater the amount of overspray generated, the more coating material consumed to coat a given item, the greater the amount of VOCs and/or HAP that are used, and the larger the quantity of total air emissions from the process.
Manual Spray Guns
Most high-end furniture is finished manually using a spray gun. Conventional air-spray application equipment atomizes the coating by mixing it with compressed air and applying it at an air pressure greater than 10 pounds per square inch (gauge) at the point of atomization. Conventional air-spray application has a TE in the range of 30 to 40 percent. However, TE as low as 20 percent is not unusual.
Automated Spray Guns
Some facilities use automated spray guns to apply coatings to either flat or three-dimensional pieces in a spray booth. Variations in item size and configuration can be handled if electronic sensors feed information about the item to a computer that controls the spray guns. TE and production rates are higher with an automated spray gun system than with manual spray guns.
Alternative Spray Guns
Other types of spray guns used in the industry include high-volume low-pressure (HVLP), airless, air-assisted airless and electrostatic. These application technologies have a higher TE than conventional air spray guns.
In automated flatline finishing, flat wood pieces travel on a conveyor to each finishing station. The system applies the coating material using rollers or an automated spray technique, or by passing the item through a curtain (cascade) of coating. Production rates are faster with flatline finishing than with manual spray gun application. In all flatline application techniques, excess coating can be collected below the conveyor and recirculated to the coating reservoir for reuse. TE in flatline finishing is significantly higher than for manual or automated spray gun application, often approaching 100 percent.
In dip coating, pieces are dipped into an open reservoir of coating material. Dip coating can be a manual or automated process. Although the TE can be high, the open reservoir generates significant air emissions if solvent-based coatings are used because the solvent can freely evaporate.
Most spray coatings are applied in a spray booth--a covered station where coating is applied in the direction of the wall that contains overspray particulate filters. When a conveyor system is used to move items through the plant, the two sidewalls have large openings to permit the entry and exit of the piece. Spray booth exhaust systems draw air from the finishing area to the atmosphere without treatment beyond particulate filters. Spray booths are the source of 85-95 percent of air emissions from finishing material application, depending on the type of coating being applied.
Spray Gun and Line Cleaning
At small facilities, the same spray gun might be used for the application of more than one type of coating, requiring gun and incoming line cleaning between each application of different types of coatings or different color coatings. At large facilities, each type of coating has its own dedicated gun and line which usually require cleaning at the end of each day or shift.
Housekeeping includes material storage and distribution, and maintenance of the facility and equipment, including routine cleaning. Cleaning solvents represent approximately ten percent of the total volume of materials purchased at a wood furniture facility.
Potential sources of air emissions from housekeeping activities include the following:
- leaks in the coating/adhesive supply system
- transfer of coatings/solvents/adhesives from large to small containers
- uncovered coating/solvent/adhesive containers
- uncovered washoff tanks
- spray booth cleaning
- coating/adhesive supply system cleaning, including spray guns
- used rags
In addition to air emissions, housekeeping activities at facilities usually generate several solid and/or hazardous waste streams.
Because of the high flammability of nitrocellulose coatings, fire codes limit the amount of material in the work site to what is needed for a single day of production. Larger quantities must be stored in a fire-resistant room in a remote area. Unless the coating transfer system is fully automated (dedicated coating lines running from storage containers to the work site), material is manually transferred from storage containers to pressure pots for use. A pressure pot is a 3- to 10-gallon pressurized container carried to the work site and connected to the spray gun. Manual transfer of materials by pouring one into the other increases aeration (and volatilization) and can result in spills that volatilize as they dry.
Sources of Solid and Hazardous Waste and Air Emissions
Sources of solid and/or hazardous wastes include spray booth cleaning, used cleaning solvents and rags, and empty coating/solvent/ adhesive containers. Also, much of the overspray dries as lacquer dust and is considered a hazardous waste because of its flammability. Additional sources of solid and/or hazardous wastes and air emissions are identified in the following diagram.
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Spray Booth Filters
Filters are paper, styrofoam, polyester, or metal and require periodic replacement or cleaning. Paper, Styrofoam, or polyester filters from spray booths (if used) are disposed of as a solid or hazardous waste, depending on the composition of the particular waste material. Cleaning of metal filters is performed using solvents to dissolve and remove the coating material. This spent solvent is disposed of as a hazardous waste.
Absorbents are often placed on the floor of the spray booth to capture liquid runoff, particularly during staining. Used absorbents are disposed of as a solid or hazardous waste, depending on the composition of the particular waste material.
Strippable Spray Booth Lining
To reduce the need to use solvents and the need for labor to remove dried coating from spray booth surfaces, spray booths are often coated with a spray-on lining that can be peeled off. Eventually, overspray builds up on the spray booth and the strippable lining is pealed off and disposed of as solid or hazardous waste depending on the composition of the waste material. In addition the strippable spray coating itself often contains a significant concentration of VOCs that are emitted as the coating is sprayed on and which then dries on the spray booth components.
For coatings that volatilize, drying between finishing steps is known as flashoff. Emissions from flashoff areas are generally uncontrolled and produce 5-15 percent of the emissions from finishing material application.
To speed drying, and therefore increase production rates, some facilities use a drying oven. Drying ovens exhaust to the atmosphere, genereally without treatment.
National Emissions Standards for Wood Furniture Manufacturing Operations-40 CFR 63 Subpart JJ.